This invention relates to a semiconductor device, such as a bipolar transistor, having at least one junction which is operated under reverse-bias in at least one mode of operation of the device.
EP-A-124139, corresponding to U.S. Pat. Nos. 4,602,266 and 4,774,560, describes a semiconductor device comprising a semiconductor body having a portion of one conductivity type adjacent one major surface, a first active device region forming with the said portion a first pn junction which terminates at the one major surface and is reverse-biassed in at least one mode of operation of the device, a second active device region provided within the first active device region and forming with the first active device region a second pn junction terminating at the one major surface, and one or more further regions of the opposite conductivity type within the said portion adjacent the one major surface and located surrounding and spaced from the first pn junction to lie within the spread of the depletion region of the first pn junction in the one mode of operation of the device to increase the reverse breakdown voltage of the first pn junction.
As described in EP-A-124139, the semiconductor device may be a vertical device and may comprise, for example, a vertical bipolar transistor with the said portion forming at least part of the collector region and the first and second active device regions forming the base and emitter regions, respectively, of the transistor. In operation of the device, the further regions act to spread the depletion region of the first pn junction laterally so as to reduce the surface electric field during the at least one mode of operation of the device, thereby increasing the breakdown voltage of the first pn junction. The further regions do, however, add to the capacitance between the said portion and the first active device region, that is the base-collector capacitance.